如何使用plotmath更新ggplot图例标签

我正在尝试更新ggplot要使用的图例标签plotmath但是，当我这样做时，它将之前组合的图例分成两部分。通过一个例子可能更容易理解：

# test data and the default plot gives the correct colours and linetypes 
# in the legend but the labels are not parsed (as have not tried to yet!)

test = data.frame(x = 1:10, y = 1:10, grp=factor(rep(c("g1", "g2"),each=5)))

ggplot(test, aes(y=y, x=x, linetype=grp, colour=grp)) +
  geom_line()  

我想要的是保留上图的图例键，但更改图例标签。我已经尝试过以下内容，现在解析plotmath但将图例键分开。

ggplot(test, aes(y=y, x=x, linetype=grp, colour=grp)) +
  geom_line() + 
  scale_linetype_discrete(breaks=c("g1", "g2"), labels = parse(text=c("g[1]", "g[2]"))) +
  scale_colour_discrete(breaks=c("g1", "g2"), labels = parse(text=c("g[1]", "g[2]"))) 


# same as before
ggplot(test, aes(y=y, x=x, linetype=grp, colour=grp)) +
  geom_line() + 
  scale_linetype_manual(values=1:2, breaks=c("g1", "g2"), labels = parse(text=c("g[1]", "g[2]"))) +
  scale_colour_manual(values=1:2, breaks=c("g1", "g2"), labels = parse(text=c("g[1]", "g[2]"))) 

如何保留基本图的键但更新标签plotmath请问（不改变原始数据）？

这适用于labels = c(~g[1],~g[2]):

ggplot(test, aes(y=y, x=x, linetype=grp, colour=grp)) +
  geom_line() + 
  scale_linetype_discrete(breaks=c("g1", "g2"), 
                          labels = c(~g[1],~g[2])) + 
  scale_colour_discrete(breaks=c("g1", "g2"),
                        labels = c(~g[1],~g[2]))